1. Technical Field
The present invention relates to a positioning system which uses signals from positioning satellites, information supply device, a terminal, a control method of the information supply device, a control program of the information supply device, and a computer readable recording medium recording the control program of the information supply device.
2. Related Art
In the past, for example, in a so-called digital mobile communication system in a CDMA (Code Division Multiple Access) system, a method to detect a position based on an arrival time difference of a receiving signal between a plurality of base stations and a mobile wireless terminal relying on the assumption that a clock synchronization is assured among the plural base stations has been known (for example, JP A-7-181242).
In such a positioning method, there is a need for time (transmission timing) when a signal is transmitted from a base station to be precise. However, a timing when a signal is transmitted from a base station, is delayed inside the base station between a time when a timing signal is generated and a time when the timing signal is transmitted, for example.
To the above mentioned situation, there is proposed a technique of installing an offset measuring instrument, obtaining offset estimation values of a transmission timing based on signals received at a plurality of monitoring points, and selecting the smallest value from among these offset estimation values to define an offset measurement value of the transmission timing of a wireless base station (JPA-2002-217824, FIG. 5, etc.).
However, inside the offset measuring instrument as well as inside the base station, there occurs a signal delay such as an antenna delay, a cable delay, or a filter delay. On the other hand, it is a general portable terminal to carry out positioning based on signals from base stations, and a signal delay inside the portable terminal is not always equal to that of the above-described offset measuring instrument.
Thus, if such a general portable terminal carries out positioning by using the offset measurement value generated by the offset measuring instrument, an error of positioning computation may be increased.